The present invention relates, in general, to oxy-fired pulverized coal combustion and, in particular, to minimizing the loss of oxygen through leakage of oxidant into the gas side of a rotary regenerative oxidant preheater.
Air quality laws, both at the federal and state level have set increasingly stringent emission standards. Often of particular concern are sulfur dioxide and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Acidic gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes.
New technologies are addressing this problem so that fossil fuels and particularly coal can be utilized for future generations without polluting the atmosphere and contributing to global warming. One of the technologies being developed has potential for retrofit to existing pulverized coal plants, which are the backbone of power generation in many countries. This technology is oxy-fuel combustion which is the process of firing a fossil-fueled boiler with an oxygen-enriched gas mix instead of air. Almost all the nitrogen is removed from the input air, yielding a stream that is approximately 95% oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion.
About 70% to 80% of the flue gas exiting the wet scrubber of an oxy-fired pulverized coal combustion plant is returned to the boiler where oxygen is introduced to produce the combustion oxidant gas, while the remainder of the flue gas is sent to a purification and compression system where it is prepared to suit pipeline and storage requirements. Thus, it is imperative that the carbon dioxide concentration be as high as possible with a low concentration of sulfur, nitrogen, oxygen, and water as can be practically and economically achieved.
Oxy-fired pulverized coal combustion burns pulverized coal in an oxidant comprised of a mixture of relatively pure oxygen and recycled flue gas to reduce the net volume of flue gases generated from the combustion process in a boiler, and to substantially increase the concentration of carbon dioxide in the flue gases. The recycled flue gas represents a portion of the flue gases generated by the combustion process and acts to dilute the flame temperature and maintain the volume necessary to ensure adequate convective heat transfer to all boiler areas, and can also be used to dry and carry the pulverized coal to the combustion space of the boiler.
The oxidant used in oxy-fired pulverized coal combustion is preferably heated in rotary regenerative type air preheaters, even though such air preheaters encounter leakage from the air side to the gas side. Tubular and plate type air preheaters do not experience leakage and provide a reasonable alternative to the rotary regenerative air preheater at industrial boiler scale. However, this is not a cost effective alternative at the electric utility boiler scale.
In conventional pulverized coal firing, a small portion of the air required for combustion is used to dry and carry the pulverized coal to the burners for burning the coal in the furnace or combustion space of the boiler. This portion of the air is known as primary air. In direct firing systems, primary air is also used to dry the coal in the pulverizer. The remainder of the combustion air is introduced in a windbox housing the burners, and is known as secondary air.
Rotary regenerative air preheaters are relatively compact and are the most widely used type for combustion air preheating in electric utility boiler plants. Rotary regenerative air preheaters transfer heat indirectly by convection as a heat storage medium is periodically exposed to heat-emitting flue gases and heat-absorbing combustion air. The rotary regenerative air preheater includes a cylindrical shell or housing that contains a coaxial rotor packed with metal heat storing corrugated plates which are bundled so as to present flow passageways therebetween. The preheater is divided into a gas side which is under negative pressure and an air side which is under positive pressure. The most prevalent flow arrangement has the flue gases entering the top of the rotor and the combustion air entering the bottom of the rotor in counter flow fashion. Consequently, the cold air inlet and the cooled gas outlet are at one end of the preheater, usually referred to as the cold end, the hot gas inlet and the heated air outlet are at the opposite end of the preheater, usually referred to as the hot end. As a result, an axial temperature gradient exists from the hot end of the rotor to the cold end of the rotor. In response to this temperature gradient, the rotor tends to distort and to assume a shape similar to that of an inverted dish, commonly referred to as rotor turndown.
In operation, the rotor is rotated slowly about a central shaft, making one to three revolutions per minute causing each bundle of heat absorbing plates to be placed, alternately, into the flow path of the heat-emitting flue gases and the flow path of the heat-absorbing combustion air. The most notable characteristic of rotary regenerative air preheaters is that a small but significant amount of air leaks from the positive pressure air side to the negative pressure gas side due to rotor turndown and the rotary operation of the air preheater. In order to prevent undue leakage from the air side to the gas side, the air preheater is provided with radial, axial and peripheral seals. It is known to construct these seals of thin, flexible metal. The seals are adjusted when the gaps are the largest. This means that, when the gaps are small due to expansion of the rotor and the housing, the seals may be severely bent and forced into high contact pressure with the rotor or housing. For this reason, seals wear relatively quickly and require replacement.
In a prior art or conventional regenerative air or oxidant preheater arrangement, the primary air or oxidant is at a positive pressure of about 40 inches of water gage (wg.), the secondary air or oxidant is at a positive pressure of about 20 inches wg, and the flue gas is at a negative pressure of about 5 inches wg. This conventional air or oxidant preheater has the air or oxidant side of the preheater divided into three sectors, a central sector which receives the primary air or oxidant and is flanked by a pair of sectors which receive the secondary air or oxidant and are located adjacent the flue gas side portion of the preheater. This arrangement minimizes the pressure difference across the seals between the air or oxidant side and the gas side to about 25 inches wg, which results in 7% to 14% leakage of air or oxidant into the flue gas. These values, though representative of a coal fired plant, may vary depending on fuel and equipment variations and are not intended as absolute.
In an oxy-fired pulverized coal plant the combustion process is carried out by the oxidant, which is comprised of a mixture of relatively pure oxygen and recycled flue gas, with a portion thereof being used to dry and transport the pulverized coal to the burners and the remainder being introduced into the boiler combustion space. The oxidant must be heated before entering the combustion process, and the equipment of choice is a rotary regenerative air preheater since it is cost effective for electric utility power plants. However, the leakage occurring in the regenerative oxidant preheater from the positive pressure oxidant to the negative pressure flue gas represents a loss of oxygen and recycled flue gas to the gas side of the regenerative oxidant preheater. This loss of oxygen along with the recycle gas requires additional oxygen production in an air separation unit to make up for the loss of oxygen, and it also requires the removal of the leaked oxygen from the product gas in a compression and purification unit before the concentrated carbon dioxide can be disposed of via storage or use for enhanced oil recovery, since pipeline line and use constraints require that the flue gas be as high in concentration of carbon dioxide and as low in concentration of nitrogen, sulfur, oxygen and water, as practical. Both of these remedial procedures result in increased plant operating costs. Thus, oxidant introduction into the flue gas must be minimized or eliminated. Furthermore, it is undesirable for an oxidant with a high concentration of oxygen to be exposed to ash potentially containing some combustible carbon and thereby raising the concern of fire.